Embodiments of the inventive concepts described herein relate to an electrode (air electrode) for lithium-air batteries and a method of manufacturing the same, and more particularly, relate a porous air electrode including a catalyst layer that is uniformly coated on the surface of individual conductive nanofibers constituting a nanofiber network having a fabric-like structure and a manufacturing method thereof.
Embodiments of the inventive concepts described herein also provide a multilayer porous air electrode having a single-layer or multilayer shell structure that is uniformly and continuously coated on the surface of the individual nanofibers and a manufacturing method thereof.
A porous air electrode obtained in embodiments of the inventive concepts has structures of (1) a single-layered core (non-conductive nanofiber)/shell (a conductive catalyst layer), (2) a multilayered core (non-conductive nanofiber)/shell (first conductive layer/second catalyst layer), (3) a single-layered core (conductive carbon nanofiber)/shell (conductive catalyst layer) or (4) a single-layered core (conductive carbon nanofiber)/shell (non-conductive catalyst layer), and thus an air electrode using current collector-catalyst monolithic nanofiber network exhibiting both current collecting and catalytic properties is provided. The porous air electrode is light, and has a large specific surface area with open structure allowing the electrolyte to penetrate easily through the pores of the nanofibers, and thus a lithium-air battery exhibiting significantly improved energy density and cycle-life characteristics is provided.
The development of sustainable and alternative energy has increasingly received attention due to the environmental problems such as an increase in energy consumption and the use of fossil fuels, and a secondary battery that can produce and store electrical energy through the charge and discharge of electricity is regarded as the most realistic solution. There are many kinds of secondary batteries like lead-acid batteries, nickel-cadmium (Ni—Cd) batteries, nickel-metal hydride (NiMH) batteries, and lithium ion (Li-ion) batteries that are currently commercialized, and in particular, lithium ion (Li-ion) batteries are most widely used in small-sized electronic devices such as mobile phones or laptop computers. However, as the secondary battery market has expanded, the development on a high-capacity energy storage device beyond the performance limit of the existing lithium-ion batteries, which can support a high output power as power sources for application in an electric vehicle (EV) or high-capacity electrical energy storage (EES) device, has been greatly desired in the recent.
Lithium-air (Li-air) batteries have recently received much attention as a next-generation energy storage system since they use lithium as the negative electrode and light oxygen as the positive electrode so as to have a much higher energy density than a lithium-ion batteries, and they use oxygen in the air as the fuel so as to have an advantage of being eco-friendly. In order to commercialize the Li-air batteries, however, the following elemental technologies are required to be solved especially in regard to the positive electrode (air electrode) which provides reaction sites to form and decompose the solid phase lithium oxide (Li2O2) as a reaction product.
First, the current collector which occupies a large weight ratio in the entire air electrode (energy density is inversely proportional to the weight or unit area of the electrode) should be lighter and the reaction area in the same size should be wide to have a large specific capacity (energy density is the multiplication of the driving voltage and the specific capacity) to obtain a high energy density. In addition, as the current collector of the commercial lithium-ion battery electrodes, a two-dimensional plate electrode such as a foil is produced from a metallic material such as copper, aluminum, nickel, or stainless steel and used, but the current collector of a Li-air batteries should have a porous structure in order to maintain a high permeability of the electrolyte containing lithium ions and an easy diffusivity of oxygen gases. Accordingly, a method to secure the current collector having an improved current density through an increase in specific surface area induced by porous and finely patterned metal mesh including various thicknesses of nickel, titanium, platinum, and the like has been attempted. As an example of this, a Li-air battery system using a metal mesh as the positive electrode has been studied in general. However, in the case the metal mesh, it has large interval between the wires from tens to hundreds μm due to the limitation of commercialized technique, and thus the specific surface area or the volumetric capacity is low. In addition, the weight of the metallic material is heavy, and thus the gravimetric energy density of the battery itself decreases when a metal mesh having a thick diameter is used. Hence, it is important to form the size of the pores in the metal mesh in the range of tens nm to several μm, and it is preferable to restrict the diameter of the individual wires constituting the metal mesh to be less than about 1 μm as well. In the case of a usual metal mesh, it may be broken or deformed due to a weak strength and the manufacture thereof is also difficult when the thickness thereof decreases to less than 1 μm, and thus it is important to select a material having an outstanding flexibility, excellent mechanical strength and lightweight property for fabricating a current collector.
Next, it is also important to select the catalysts to facilitate the OER (oxygen evolution reaction) and ORR (oxygen reduction reaction) for having long-cycle-life of Li-air batteries. Unlike a lithium-ion batteries, lithium and oxygen meet to form solid lithium oxide (Li2O or Li2O2) during discharge in non-aqueous Li-air batteries, and such the reaction products should be reversibly decomposed into lithium and oxygen during charge, and thus the use of efficient catalysts is essential for achieving high round trip efficiency in repeated charging and discharging steps. Among the well-known catalysts, gold and platinum exhibit the best catalytic performance, but these even decompose the electrolyte during charge and discharge, resulting in poor cycle-life of the Li-air batteries. In addition, the high price of the noble metal catalysts such as gold or platinum still inhibit the commercialization of the Li-air batteries in the large scale EV application in which the large amount of the catalysts are required. So, the use an inexpensive catalyst that can maintain a stable structure while having excellent catalytic activities is required, and a catalyst material of binary transition metal oxides such as ruthenium oxide (RuO2), iridium oxide (IrO2), cobalt oxide (CO3O4), or manganese oxide (MnO2) or a perovskite-based three-component oxides such as lanthanum-manganese oxide (LaMnO3) or lanthanum cobalt oxide (LaCoO3) has been widely studied. Besides oxides above, nitride- or carbide-based catalysts including titanium nitride (TiN) or titanium carbide (TiC) have been recently extensively studied. However, the cycle-life of the batteries using such catalysts is significantly low to have less than 100 cycles as reported so far. Hence, it is important to provide a catalyst capable of providing more stable reactivity and a three-dimensional current collector having a porous structure for effective penetration of oxygen and lithium ions at the same time for the development of Li-air batteries exhibiting excellent long-cycle-life characteristics.
Recently, carbon-based materials such as carbon black, Super P, Ketjen black, carbon nanotubes (CNTs), or graphene are widely used in the air electrode for the purpose of providing a large specific surface area and high electrical conductivity. However, the carbon-based materials produce a side reaction product such as lithium carbonate (Li2CO3) in the electrolyte during repeated charge and discharge process. It is required to apply a significantly high voltage (4.2 V or higher based on reduction potential of lithium) to decompose such a side reaction product, but it also cause the electrolyte decomposition in such a high voltage condition.
Finally, it is desired to secure the mass production and the price competitiveness for the commercialization of the Li-air batteries, and all the positive electrode materials are desired to have high physical or chemical stability so that they are not deformed or subjected to corrosion during the charge and the discharge reaction. A development of a new air electrode that both a current collector and a catalyst are integrated is desired to solve all the above problems.